1. Field of the Invention
The invention relates in general to method of converging an exposure value (EV), and more particularly, to a method of establishing a lookup table of exposure values to fast converge an appropriate exposure value.
2. Related Art of the Invention
In a digital photographic device with 1024xc3x97768 pixels and a brightness range of 0 to 255 for each pixel, how to fast converge the average brightness of an image has become the most exigent problem to be resolved.
In Taiwanese Patent No. 234748 xe2x80x9cFuzzy controlled automatic exposure system of digital electrostatic camera,xe2x80x9d a fuzzy control method has been used to maintain appropriate brightness of a captured image. However, the fuzzy control method is so complex that an experienced professional operator is often required to regulate the control mechanism, so as to maintain the appropriate brightness of the image. Therefore, the automatic convergence process of exposure value is too slow, and consumption of computation resources is significant.
The present invention provides a method of fast converging an appropriate exposure value. In the present invention, a lookup table of exposure values that is easily modified is established. An image captured by an optical sensor of a digital photographic device can thus be fast converged into an appropriate exposure value in a close-loop computation. Therefore, the present invention not only easily achieves multi-variable exposure control, including the control of exposure time, gain value and f-number, but also fast converges an appropriate automatic exposure value.
The fast converging method provided by the present invention is suitable for use in a digital photographic device and comprises the following steps. A lookup table of exposure values is established. In the lookup table, the exposure value is a function of exposure time, gain value and F-number. An automatic exposure convergence window is defined, and a center of the automatic exposure convergence window is defined as a target brightness. A previous exposure value is selected as an initial setup value. An image is captured by an optical sensor of the digital photographic device and segmented into a plurality of sub-regions. The average brightness of each sub-region is multiplied by a corresponding image weight and divided by an overall image weight, such that an average brightness of the image is obtained. A predicted exposure value is obtained according to the previous exposure value and a truncate function, which is a function of the target brightness and the average brightness. When the predicted exposure value is the same as the previous exposure value, the automatic value is convergent and the automatic exposure is achieved. Meanwhile, the predicted exposure value is the appropriate exposure value. If the predicted exposure value is not equivalent to the previous exposure value, the predicted exposure value is used as the previous exposure value. The exposure time, gain value and f-number of the previous exposure value can be obtained from the lookup table of exposure values, and the steps of capturing the image, segmenting the image, obtaining the predicted exposure value and automatic exposure are repeated until the predicted exposure value is automatically converged.
In one embodiment of the present invention, the table of exposure values can be adjusted.
In one embodiment of the present invention, the minimum interval of exposure values in the lookup table is referred to for the resolution of exposure value, which can be adjusted.
The dimension of the automatic exposure convergence window is proportional to the resolution of exposure value.
In the present invention, the dimension of the automatic exposure convergence window is equivalent to the resolution of exposure value. The automatic exposure convergence window is located between the target brightness times two to the power of negative resolution of exposure value divided by two and the target brightness times two to the power of resolution of exposure value divided by two.
In one embodiment, the predicted exposure value is derived by subtracting the truncate function from the previous exposure value.
In one embodiment of the present invention, the optical sensor includes a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device.
According to the above, the present invention establishes an easily modified table of exposure value, allowing an appropriate exposure value of an image captured by an optical sensor of a digital photographic device to be fast converged by a close-loop computation. Therefore, the prevent invention achieves the multi-variable (exposure time, gain value and fF-number) exposure control, and fast converges the appropriate automatic exposure value.